Background to drainage exceedance

It is inevitable that as a result of heavy or extreme rainfall the capacities of sewers and other drainage systems will be exceeded on occasion. Drainage exceedance will occur when the rate of surface water runoff exceeds the inlet capacity of the drainage system, when the receiving water or pipe system becomes overloaded, or when the outfall becomes restricted due to flood levels in the receiving water.

Underground conveyance cannot economically or sustainably be built large enough for all types of extreme rainfall, as a result, there will be occasions when surface water runoff will exceed the capacity of drains. This is especially problematic with combined sewers (combining foul and surface water) and sewage flooding can result. When drainage system capacity is exceeded the excess water (exceedance flow) surcharges above ground, causing surface water flooding and indiscriminate flooding of property can occur when this flow of water is not controlled.

Although designers of drainage in new developments have to consider extreme rainfall in their designs, there is no obligation to properly manage the consequence of such events. Sewers for adoption and the National Planning Policy technical guidance identifies that overland flood pathways should be considered, but no recommendation of the level of protection is given.

Designing for exceedance can make the most of shared spaces and create multi-functional infrastructure. We may use highways to channel water, raise or drop kerbs to redirect water, and use car parks and green open space to store it. Property level flood protection and flood walls can prevent water entering properties. These measures can be used when retrofitting to reduce the risk of flooding happening in the future. In new development and through regeneration, there are more opportunities to change the topography of the landscape. This makes it easier to design for exceedance.

Designing for exceedance requires different disciplines, people and organisations to work together in partnership, with each being aware of the role that the other plays. This applies to both new development and retrofitting. In new development, spatial planners, urban designers, landscape architects and architects are the primary influencers of the design. Considering drainage and exceedance from the start often makes it easier to accommodate minor changes to create an area with a lower surface water flood risk. When retrofitting, drainage engineers may be the first to be involved, before a wider range of disciplines help develop the solution.

Above ground conveyance, flood pathways and flood routing is essential in allowing runoff from extreme rainfall events to drain from developed areas effectively. Recognising the importance of flood pathways along highways and other routes, and the storage of water in low spots, is the first step to better urban flood management. Through good urban and drainage design, a second important step is to direct flood flows along routes where the risk of property flooding and the risk to health and safety is minimal and can be managed. Options to achieve this are available, and explored within this website and in more details in CIRIA C635 Designing for exceedance. Further outputs are also available from CIRIA C738 Managing urban flooding from heavy rainfall - encouraging the uptake of designing for exceedance that provides case studies, a reccomendations and summary report, a lessons and success factors report and a literature review.

With careful design and good stakeholder engagement highways and other urban features can be effectively used to convey exceedance flows. Relatively minor features of the urban landscape, such as kerb heights, traffic calming and property threshold details can significantly affect flood risk.

Engaging stakeholders to collectively manage and maintain flood routes, and designing buildings to be more flood resistant, is another important factor in the equation.